Generally, a semiconductor wafer such as a silicon substrate is subjected to various processes including a film forming process, an etching process, an oxidation process, a diffusion process, and a modification process to fabricate a semiconductor integrated circuit. For example, the oxidation process among those processes is used for oxidizing a surface of a single-crystal silicon film or polysilicon film and for oxidizing a metal film. The oxidation process is used mainly for forming gate oxide films and insulating films for capacitors.
Oxidation methods are classified by pressure into atmospheric pressure oxidation methods that are carried out in an atmospheric atmosphere and vacuum oxidation methods that are carried out in a vacuum atmosphere. Oxidation methods are classified by oxidizing gas into wet oxidation methods including a wet oxidation method disclosed in, for example, Japanese Patent Laid-Open Publication No. 140453/1991, that use steam generated by burning hydrogen in an oxygen atmosphere in an external combustor, and dry oxidation methods including a dry oxidation method disclosed in, for example, Japanese Patent Laid-Open Publication No. 1232/1982 that supply only ozone or oxygen into a processing vessel.
In view of quality and characteristics including dielectric strength, corrosion resistance and reliability, an insulating film formed by a dry oxidation process is superior to that formed by a wet oxidation process. In view of deposition rate and intrafilm uniformity, generally, an oxide film (insulating film) formed by an atmospheric oxidation process is satisfactory in oxidation rate but the same is not satisfactory in the intrafilm thickness uniformity of an oxide layer formed on the surface of the wafer. On the other hand, an oxide film formed by a decompressed wet oxidation process is satisfactory in the intrafilm thickness uniformity of the oxide layer but the same is not satisfactory in oxidation rate.
Design rules that have been hitherto applied to designing semiconductor integrated circuits have not been very sever the aforesaid various oxidation methods have been selectively used taking into consideration purposes of oxide films, process conditions and equipment costs. However, line width and film thickness have been progressively decreased and severer design rules have been applied to designing semiconductor integrated circuits in recent years, and design rules requires higher film characteristics and higher intrafilm thickness uniformity of films. The conventional oxidation methods are unable to meet such requirements satisfactorily.
A wet oxidation system disclosed in, for example, Japanese Patent Laid-Open Publication No. 18727/1992 supplies H2 gas and O2 gas individually into a lower region in a vertical quartz reactive tube, burns the H2 gas in a combustion space defined in a quartz cap to produce steam, makes the steam flow upward along a row of wafers to accomplish an oxidation process. Since this prior art oxidation system burns H2 gas in the combustion space, a lower end region in the processing vessel has a high steam concentration, the steam is consumed as the same flows upward and an upper end region in the processing vessel has an excessively low steam concentration. Accordingly, the thickness of an oxide film formed on the surface of the wafer is greatly dependent on the position where the wafer is held for the oxidation process and, in some cases, the interfilm thickness uniformity of the oxide film is deteriorated.
Another oxidation system disclosed in, for example, Japanese Patent Laid-Open Publication No. 1232/1982 arranges a plurality of semiconductor wafers in a horizontal batch-processing reaction tube, supplies O2 gas or supplies O2 gas and H2 gas simultaneously through one of the opposite ends of the reaction tube into the reaction tube, and forms an oxide film in a decompressed atmosphere. However, since this prior art oxidation system forms a film in an atmosphere of a relatively high pressure by a hydrogen burning oxidation method, steam is a principal element of reaction, an upper region with respect to the flowing direction of gases in the processing vessel and a lower region in the processing vessel differ excessively from each other in steam concentration and hence it is possible that the interfilm thickness uniformity of the oxide film is deteriorated.
A third oxidation system disclosed in, for example, U.S. Pat. No. 6,037,273 supplies O2 gas and H2 gas into the processing chamber of a wafer-fed processing vessel provided with a lamp heating device, makes the O2 gas and the H2 gas interact in the vicinity of the surface of a semiconductor wafer placed in the processing chamber to produce steam, and forms an oxide film by oxidizing the surface of the wafer with the steam. However, this oxidation system supplied O2 gas and H2 gas through gas inlets spaced a short distance in the range of 20 to 30 mm from the wafer into the processing chamber, makes the O2 gas and the H2 gas interact in the vicinity of the surface of the semiconductor wafer to produce steam, and forms the oxide film in an atmosphere of a relatively high process pressure. Thus, it is possible that the intrafilm thickness uniformity of the film is deteriorated.
In order to solve the above disadvantages, Japanese Patent Laid-Open Publication No. 2002-176052 disclosed by the Applicant supplies simultaneously an oxidative gas such as O2 gas and a reductive gas such as H2 gas to the upper and lower regions of a process chamber, makes the gases interact in a vacuum atmosphere to form an atmosphere mainly including active oxygen species and active hydroxyl species, and oxidizes a silicon wafer or the like in this atmosphere.
The oxidizing method is described in brief with reference to FIG. 8. FIG. 8 is a schematic structural view of a conventional oxidation system. As shown in FIG. 8, the oxidation system 2 includes a tubular processing vessel 6 of a vertical type, and a resistive heater 4 disposed on an outer periphery of the processing vessel 6. The processing vessel 6 has a wafer boat 8 which is capable of being vertically loaded and unloaded from a lower side of the processing vessel 6. Semiconductor wafers W such as silicon substrates are mounted and held on the wafer boat 8 in a tier-like manner. An H2 gas nozzle 10 for supplying H2 gas and an O2 gas nozzle 12 for supplying O2 gas are disposed on a lower part of a sidewall of the processing vessel 6. An exhaust port 14 connected to a not-shown vacuum pump is disposed on an upper part of the processing vessel 6.
The H2 gas and the O2 gas are introduced from the nozzles 10 and 12 to the lower part of the processing vessel 6. The H2 gas and the O2 gas interact with each other in the processing vessel 6 under pressure of less than 133 Pa, while generating active oxygen species and active hydroxyl species. These species move upward in the processing vessel 6, while contacting surfaces of the wafers W to oxidize the same.
According to the oxidation methods disclosed in the above respective documents, an oxide film of a suitable quality can be formed, as well as a high intrafilm thickness uniformity of the oxide film can be maintained. However, in the oxidizing method disclosed by Japanese Patent Laid-Open Publication No. 2002-176052 which uses active species, there is a problem in that an interfilm thickness uniformity is significantly unsatisfactory, which indicates a difference of the film thicknesses of the wafers. This may be because a concentration of the active species is higher on the upstream of the gas flow, while it becomes low on the downstream of the gas flow. To be specific, the interfilm thickness uniformity is dependent on the silicon surface area to be oxidized. For example, when a lot of dummy wafers each having a sufficiently thick (e.g., 100 nm-1 μm) oxide film thereon are loaded in a furnace, less active species are consumed. On the other hand, when the silicon surface area to be oxidized is relatively large, more active species are consumed, provided the same concentration of the active species exist in the furnace. Similarly, an amount of consumption of the active species depends on a size of the silicon surface of a semiconductor integrated circuit which is subjected to an atmosphere of the active species. (This is referred to as “loading effect”.) The loading effect causes a significant deterioration of the interfilm thickness uniformity of the oxide films. Conventionally, a temperature distribution in a furnace has been varied in order to compensate for lower concentration of active species. That is, a temperature adjustment is performed by increasing the temperature at a location where the film thickness is small (i.e., on the downstream of the gas flow) to accelerate an oxidation, so that a uniform film thickness distribution can be obtained in the furnace. Specifically, a temperature gradient of 10° C. to 100° C. is provided in a longitudinal direction of the furnace so as to secure the uniform thicknesses of the wafers (wafers arranged in the longitudinal direction of the furnace).
In recent years, since semiconductor integrated circuits are used for various applications, there is a tendency that many kinds of semiconductor integrated circuits are manufactured in small amounts. The wafer boat 8 can contain up to about 50 to 150 pieces of product wafers as a maximum capacity. However, it is possible that a quantity of product wafers less than the maximum capacity of the wafer boat 8, are contained in the wafer boat 8. Since the wafer boat 8 is not fully filled, dummy wafers are contained in a vacant space for the absent wafers to fill the wafer boat 8, in order not to give harmful effects to a temperature distribution and a gas flow in the processing vessel 6, when an oxidation treatment is carried out. Thus, the distribution and amount of the active species are largely varied depending on a containing condition of the product wafers and the dummy wafers in the wafer boat. Thus, a disadvantageous effect may occur that high interfilm thickness uniformity is not maintained.